1. Field of the Invention
The present invention involves a method for determination of degree of molecular dissociation in plasma using combined techniques of electrostatic measurement and emission spectroscopy.
2. Information Disclosure Statement
Continuous plasmas produced by direct current, radio frequency (rf) or microwave generators are becoming increasingly important. Such plasmas can be used to coat, clean or modify the surfaces of metals, ceramics, glasses and plastics. All these technical processes work best under certain well defined parameters such as pressure, input power and gas composition.
One particularly important plasma parameter and the one addressed by this patent is the degree of dissociation of the gas molecules. Currently this parameter is determined non-invasively by measuring the fluorescence response signal resulting from laser excitation of the plasma. Selecting the laser wavelength essentially allows the selective excitation of characteristic transitions in atoms, molecules and radicals of interest. The observed subsequent fluorescence at emission wavelengths characteristic of the species allows the determination of the degree of dissociation. However this technique requires expensive lasers and optical equipment. Emission spectroscopy is a well understood and comparatively cheap technique which is used extensively to obtain information on the chemical species present in plasmas. Electrostatic probes are also used mainly in research devices to obtain information on the electron energy distribution function (EEDF) in plasmas and are also well understood and comparatively cheap.
The EEDF can be measured using an electrostatic probe technique. The probe consists of a small piece of metal, usually cylindrical, which is exposed to the plasma. As a voltage is applied to this probe, the current collected from the discharge can be related to the energies of the charged species present. From an analysis of this current-voltage (I-V) characteristic many of the plasma parameters can be determined, including the EEDF. This type of analysis can now be automated, both the collection of the I-V characteristic and its interpretation to obtain the plasma parameters and EEDF (mainly the calculation of the first and second derivative), [Hopkins and Graham 1986].
In general, the use of an electrostatic probe in a continuously operating plasma is straight forward. In rf driven discharges care must be taken to ensure that the rapid time dependant changes in the plasma potential do not influence the measurement. There is a recent thorough review of the problems of measuring EEDF's in rf discharges, [Godyak et al. 1992].
For operation in rf discharges we propose the use of a compensated probe technique. Here, an ac voltage derived from the plasma is applied to the probe to compensate for the voltages generated by the radio frequency, including fluctuations. The measurement and analysis of the I-V characteristic is then the same as in continuously operated discharge.
It should be noted that most plasmas of technical interest today have non-thermal (non-Maxwellian) EEDF's and mean electron energies in the range 0.5 to 2 eV. This is much lower than the 10 eV typically required to cause significant dissociation of molecular species. For these plasmas, neither emission spectroscopy nor electrostatic probes provides quantative information on the degree of dissociation of the molecular species.